In recent years, progress has been made in research and development of diverse functional devices which involve use of an organic semiconductor.
One typical example of such functional devices is an organic EL device. The organic EL device, which is a current-driven light-emitting device, includes a pair of electrodes, i.e. an anode and a cathode, and a functional layer layered between the pair of electrodes, the functional layer containing an organic material. The functional layer includes a light-emitting layer, a buffer layer and the like. A hole injection layer for facilitating injection of holes may be inserted between the functional layer and the anode.
To drive the organic EL device, a voltage is applied to between the pair of electrodes to use the phenomenon of electroluminescence that occurs when holes injected from the anode into the functional layer recombine with electrons injected from the cathode into the functional layer. Being self-light-emissive, the organic EL device is highly visible. In addition, being a complete solid-state device, the organic EL device has excellent impact resistance. Owing to these advantages, more attention is being given to the applications of organic EL devices as a light-emitting device or a light source for various display apparatuses.
The organic EL device is roughly classified into two types depending on the type and forming method of the material of the functional layer. The first type is a vapor-deposition-type organic EL device in which a low molecular material is used as the main material of the functional layer, and a film of the functional layer is formed in a vacuum process such as a process by the vapor deposition method. The second type is a solution-deposition-type organic EL device in which a high molecular material or a low molecular material having excellent film formability is used as the material of the functional layer, and a film of the functional layer is formed in a wet process such as a process by the ink jet method or the gravure printing method.
Among these, the vapor-deposition-type organic EL device of the first type has been developed with priority due to high luminous efficiency of its light-emitting material having and its driving longevity, and has already been put into practical use in displays for mobile phones, small-scale televisions or the like. On the other hand, the solution-deposition-type organic EL device of the second type is effective in realizing a large-scale organic EL device, and the development has been continued as well.
Meanwhile, a variety of researches and developments are also underway for improving the luminous efficiency of the organic EL device. The targets of such researches and developments include the hole injection layer that, inserted between the functional layer and the anode, greatly contributes to the improvement of the luminous efficiency. Conventionally, it was common that the hole injection layer is formed from an organic material such as copper phthalocianine or PEDOT (electroconductive polymer). In recent years, however, attempts have been made to form the hole injection layer from a metal oxide such as nickel oxide or tungsten oxide (see Patent Literatures 1 and 2).
Among all, it has been reported that the organic EL device containing the hole injection layer made of a p-type metal oxide, such as nickel oxide, improves the hole injection efficiency and the luminance (see Non-Patent Literatures 1 and 2). In particular, it has been reported that, when the hole injection layer made of nickel oxide contains excess oxygen in the composition ratio and thus an increased proportion of Ni3+, it can produce an excellent luminance (see Patent Literature 1 and Non-Patent Literature 3).